Memory devices are known in the art for storing data in a wide variety of electronic devices and applications. More recently, SONOS (Silicon Oxide Nitride Oxide Silicon) type memory devices have been introduced. SONOS type flash memory cells comprise a gate stack having a gate layer situated over an ONO (Oxide Nitride Oxide) stack. The gate stack is situated over a semiconductor substrate where a channel region is defined between first and second terminals regions in the semiconductor substrate, thereby forming a transistor.
The ONO stack comprises a non-conductive dielectric layer, typically a silicon nitride layer (“nitride layer”), positioned between two silicon oxide layers. The nitride layer functions as an electric charge storing medium. Moreover, the nitride layer is capable of locally storing an electrical charge on one side of the nitride layer independent of an electrical charge stored on an opposite side of the nitride layer. Thus, SONOS type memory cells can be described as capable of storing two binary bits, e.g., a left bit and a right bit.
Conventional techniques for forming the nitride layer of the ONO stack produce a number of negative effects, which detrimentally affect the performance of the memory device. Typically, the nitride layer is formed using a chemical vapor deposition (“CVD”) process with a precursor consisting of silicon hydride (SiH4) (“silane”) and ammonia (NH3), or dichlorosilane (SiH2Cl2) (“DCS”) and ammonia. During the CVD process, the nitrogen-hydrogen bond in ammonia and/or the silicon-hydrogen bond in silane or DCS desirably break. When these bonds break, the hydrogen atoms react with each other to form stable H2 molecules which are pumped out of the reaction chamber. However, a substantial number of the nitrogen-hydrogen bonds and/or the silicon-hydrogen bonds do not break and will remain in the nitride film of the ONO stack. As a consequence, the resulting nitride layer will have substantial hydrogen content, typically in the range of about 1 to 2 atomic percent. The sizable content of hydrogen in the nitride layer becomes detrimental, for example, when energetic electrons are injected into the nitride layer during subsequent programming cycles. These electrons may break the nitrogen-hydrogen bonds and/or the silicon-hydrogen bonds in the nitride layer, freeing a substantial number of hydrogen atoms (“hydrogen radicals”). The presence of hydrogen radicals in the nitride layer produces a charge loss in the nitride layer, resulting in such negative effects as a shift in the threshold voltage of the memory cell, which results in unpredictable memory device behavior. Furthermore, the charge loss in the nitride layer may further result in the loss of programming data and/or programming capability in the memory cell. The hydrogen radicals may also migrate into the adjacent oxide layers, such as the top and bottom layers of the ONO stack and further degrade device properties. These negative effects result in poor performance of the memory device.
Accordingly, there exists a strong need in the art for a memory cell structure and method for fabricating a memory cell structure having a nitride layer with significantly reduced charge loss.